Maybe Lua can do this,
set_by_lua $notused '
ngx.var.keepalive_timeout = ngx.var.kealtime;
';
https://github.com/chaoslawful/lua-nginx-module
set_by_lua $notused '
ngx.var.keepalive_timeout = ngx.var.kealtime;
';
https://github.com/chaoslawful/lua-nginx-module